1. Field of the Invention
The present invention relates to a discharge lamp starting device for a fluorescent lamp which employs a semiconductor switch and a nonlinear capacitor.
A nonlinear capacitor has been developed by TDK Electronics Co., Ltd. (Tokyo Denkikagaku Kogyo Kabushiki Kaisha) which utilizes a polycrystal of BaTiO.sub.3 to provide a desired nonlinearity. This nonlinear capacitor is briefly described in "Journal of Electronic Engineering", March 1980, page 20.
2. Description of the Prior Art
An example of a conventional discharge lamp starting device using a nonlinear capacitor and semiconductor switch is shown in FIG. 1. With this device, the discharge lamp can be started substantially instantaneously. In FIG. 1, reference numeral 1 designates a fluorescent lamp, 2 an inductive stabilizer, 3 a semiconductor switch or a bi-directional diode thyristor (silicon symmetrical switch--SSS), 4 and 5 diodes, 6 a resistor, 7 a nonlinear capacitor, and U and V power source terminals. FIG. 2 shows the waveform of the voltage developed across the lamp 1.
In the circuit shown in FIG. 1, an AC voltage e as indicated by the dotted line in FIG. 2 is applied between the power source terminals U and V. In the initial starting period, when the supply voltage e reaches the breakdown voltage of the thyristor 3 at a phase angle .theta..sub.1, the thyristor 3 is turned on as a result of which current is allowed to flow through the stabilizer 2 to preheat the filaments of the lamp 1. The preheating current becomes zero at a phase angle .theta..sub.2 and the thyristor 3 is turned off. At this point, the voltage of the capacitor 7 is zero and the supply voltage e is close to the negative peak value. After the supply voltage e reaches the negative peak value, the capacitor 7 is charged with the polarity indicated in FIG. 1. The capacitor 7 has a saturable characteristic with the relation between the voltage V and the charge Q being as shown in FIG. 3. IF the capacitor characteristic is selected so that the capacitor voltage is in the nonlinear region when the supply voltage is lower than the peak value, the charging current to the capacitor 7 is abruptly decreased when the voltage reaches the nonlinear region. Due to the inductive stabilizer 2, the voltage of the charged capacitor abruptly increases. That is, a pulse voltage V.sub.1 shown in FIG. 2, which has a peak value much higher than the supply voltage peak value, is applied to the lamp 1.
After the pulse voltage has been generated, the supply voltage 2 is applied to the lamp until the thyristor 3 is turned off again. This condition is maintained unchanged until the lamp 1 has been started. When the lamp 1 has been started, the lamp voltage becomes lower than the supply voltage e. In addition, because of the action of the resistor 6, the charging current to the capacitor 7 when the lamp voltage is in the positive direction is decreased and the lamp voltage in the positive direction becomes lower than the threshold voltage of the thyristor 3. Thus, a stable discharge operation is maintained in the lamp 1. In this case, the capacitor 7 is charged to the polarity as illustrated in FIG. 1 through the diode 5 when the lamp voltage is in the negative direction so that the lamp voltage is increased. However, the lamp is maintained discharged due to the diode 4.
As is apparent from the above description, the threshold voltage V.sub.BO of the diode thyristor 3 must be higher than the lamp voltage V.sub.3 after the discharge and lower than the supply voltage peak value V.sub.4. However, if the threshold voltage is high enough to approach the peak value V.sub.4, then the firing phase angle .theta..sub.1 of the thyristor 3 lags and thereby decreases the preheating current. Accordingly, the range of V.sub.BO actually required for the diode thyristor 3 is smaller than V.sub.3 -V.sub.4. However, a diode thyristor having such a small range of V.sub.BO is considerably expensive, and accordingly a discharge lamp lightning device using such a diode thyristor is also expensive.